In most of the high energy particle detection field and the medical imaging field such as computed tomography (abbreviated as CT), positron emission tomography (abbreviated as PET) and single photon emission computed tomography (abbreviated as SPECT), the scintillation pulse signal collected and processed by the data acquisition system is an observable electric signal obtained by converting visual light by a photoelectric conversion device, and the visual light is obtained by converting high energy particles (such as y ray and X ray) by a scintillation crystal. A typical scintillation pulse waveform is as shown in FIG. 1. Time information of the scintillation pulse is obtained by measuring the time of a relatively-fixed point on the pulse. Energy information of the scintillation pulse is obtained by calculating the total amount of the electric charges carried by the pulse, i.e., the area of the pulse waveform. Position information of the scintillation pulse is the relative position (X, Y) of the scintillation pulse on the detector obtained by comparing four “angular signals” generated by the detector.
In a traditional scintillation pulse data acquisition system, the information obtaining is based on the analog circuit or the analog-digital hybrid circuit. The high-speed scintillation pulse signal needs to be processed by analog-amplifying, filtering, integration and the like, and drift may occur for the analog circuit as the changing of the temperature and the time, therefore, it is difficult to maintain the performance of the detector in an optimum state. In addition, the analog-amplifying, filtering and integration are performed according to specific characteristics of certain detector, therefore, the traditional scintillation pulse information obtaining method has poor adaptability to different detectors.
Most of the existing digital scintillation pulse information obtaining methods for scintillation pulse obtaining are based on the analog-to-digital convertor (abbreviated as ADC). Because the rising time of the scintillation pulse is generally between 1 ns and 10 ns, and the decay time constant is generally between 10 ns and 300 ns (depending on the type of the detector), the sampling speed of the ADC is required to be more than 1 GHz for acceptable time resolution, and the sampling speed of the ADC is required to be more than 200 MHz for acceptable energy resolution and space resolution. Also, the high sampling rate ADC requires a high processing speed and a high transmission bandwidth, which makes the design of the data obtaining system difficult. In the existing digital scintillation pulse data acquisition system, some analog circuits for filtering and shaping are still needed to convert a high-speed scintillation pulse into a low-speed signal, and the sampling is performed by a lower-speed ADC. Therefore, an all-digital data acquisition system based on the ADC for scintillation pulse sampling can not be achieved by existing technologies.
Currently, a method and a device for gamma photon detection are provided (U.S. Pat. No. 7,199,370B2). Energy, peak time and a decay time constant can be obtained by using this method without an ADC. In this method, two reference voltages Vi and Vj are setup in advance with Vj<Vi, time difference tij between the time when the falling edge voltage of the pulse is Vi and the time when the falling edge voltage of the pulse is Vj is measured, and the decay time constant T of the scintillation pulse may be calculated by the formula:T=tij/In(Vi/Vj).Then, two reference voltages Vk and Vi are set in advance, the time period tk during which the amplitude of the pulse voltage is larger than Vk and the time period tl during which the amplitude of the pulse voltage is larger than Vi are measured, and the peak amplitude Vp of the scintillation pulse may be calculated by the formula:
            V      p        =                            V          k                                      s            +            1                    s                    ⁢      exp      ⁢              {                                                            (                                  s                  +                  1                                )                            ⁢                              t                k                                      -                          t              l                                sT                }              ,where s=Vl/Vk−1, and Vp may represent a relative value of the pulse energy. Then, a reference voltage Vm is set in advance, the time period tm, between the time when the rising edge voltage of the pulse is Vm and the time when the rising edge voltage of the pulse is Vi is measured, and the peak time tp of the scintillation pulse may be calculated by the formula:tp=(Vi/(Vi−Vm))tmi.However, this method has the following three disadvantages: (1) the time measured in the method is a time period between two points on a pulse, which is not the absolute time of the two points, therefore, the peak time tp of the pulse obtained in the method only represents the relative time of the whole pulse, that is, in which time period of the pulse the peak occurs, instead of representing the absolute time the pulse occurs, (2) the position information of the scintillation pulse can not be obtained in the method, (3) the pulse energy is obtained with a big error since only two reference voltages are used to acquire the pulse energy in the method. In view of the above, a digital scintillation pulse data acquisition system can not be achieved by using the method independently.
A new sampling method, which is an MVT sampling method based on a time sampling principle, is proposed by Qingguo Xie etc. in 2005. In the MVT sampling, the time is sampled with a given sampling voltage to acquire a sampling point, different from the ADC sampling in which the voltage is sampled with a given sampling time.
By sampling the rising edge of the scintillation pulse using the MVT sampling method and performing linear fitting on the obtained sampling points, the time information (Qingguo Xie, Chien-Min Kao, Xi Wang, Ning Guo, Caigang Zhu, Henry Frisch, William W. Moses and Chin-Tu Chen, “Potentials of Digitally Sampling Scintillation Pulses in Timing Determination in PET,” IEEE Trans. Nucl. Sci., Vol 56, Issue 5, pp. 2607-2613, 2009) and the energy information in certain energy spectrum range (H. Kim, C. Kao, Q. Xie, C. Chen, L. Zhou, F. Tang, H. Frisch, W. Moses, W. Choong, “A multi-threshold sampling method for tof-pet signal processing,” Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment Vol. 602, Issue 2, pp. 618-621, 2009) of the original pulse may be obtained. However, in the published two methods, the number of threshold voltages is small, the setting method is simple, and all of the sampling points are used for calculation, thereby leading to an obvious defect which is that the scintillation pulse in the whole energy spectrum range can not be accurately measured with finite threshold voltages, especially when the amplitude of the scintillation pulse is small. The position information of the scintillation pulse is to be obtained by dividing a scintillation pulse with a normal amplitude into four scintillation pulses (angular signals) with different sizes by a resistance network and comparing the proportions of the amplitudes (energy) of the four scintillation pulses, but the position information of the scintillation pulse can not be acquired by using the published two methods since all the amplitudes of the four pulses are small.
A typical scintillation pulse waveform is as shown in FIG. 1, and the waveform includes a rising edge which rises rapidly and a falling edge which drops slowly. The rising speed of the rising edge depends on the scintillation crystal and the photoelectric conversion device, and the decay speed of the falling edge depends on the characteristic of the scintillation crystal.
Regardless of noise, a single scintillation pulse model may be expressed in multiple ways. Generally, the single scintillation pulse is considered as an ideal signal model including a rising edge which rises linearly and a falling edge which drops exponentially. The ideal scintillation pulse waveform is as shown in FIG. 2, and the waveform model is expressed as equation (1):
                              V          ⁡                      (            t            )                          =                  {                                                    0                                                              t                  <                                      -                                                                  Line                        ⁢                                                                                                  ⁢                        B                                                                    Line                        ⁢                                                                                                  ⁢                        K                                                                                                                                                                                      Line                    ⁢                                                                                  ⁢                    K                    ×                    t                                    +                                      Line                    ⁢                                                                                  ⁢                    B                                                                                                                    -                                                                  Line                        ⁢                                                                                                  ⁢                        B                                                                    Line                        ⁢                                                                                                  ⁢                        K                                                                              ≤                  t                  <                  tp                                                                                                      exp                  ⁡                                      (                                                                  Exp                        ⁢                                                                                                  ⁢                        K                        ×                        t                                            +                                              Exp                        ⁢                                                                                                  ⁢                        B                                                              )                                                                                                t                  ≥                  tp                                                                                        (        1        )            where LineK is the slope of the straight line of the rising edge and LineK>0, LineB is the intercept of the rising edge which may be an arbitrary value and has a linear relationship with the starting time of the rising edge, ExpK is a decay time constant and ExpK<0, the parameter ExpB may be an arbitrary value and has a linear relationship with the starting time of the falling edge, and tp is the peak time of the pulse. Therefore, an ideal scintillation pulse is expressed by four parameters LineK, LineB, ExpK and ExpB. Information such as the starting time, the peak time, the peak amplitude, the energy and the decay constant of the scintillation pulse signal may be calculated from these four parameters by the following formulas:
(a) the starting time of the pulse t0
                              t          ⁢                                          ⁢          0                =                  -                                    Line              ⁢                                                          ⁢              B                                      Line              ⁢                                                          ⁢              K                                                          (        2        )            
(b) the peak time tp, an approximate solution may be obtained by solving the equation (3),LineK×t+LineB=exp(ExpK×t+ExpB)  (3)
(c) the peak amplitude VpVp=LineK×tp+LineB  (4)
(d) the energy E
                    E        =                              ∫                                          V                ⁡                                  (                  t                  )                                            ⁢              dt                                =                                                                      Line                  ⁢                                                                          ⁢                  K                  ×                  tp                                +                                  Line                  ⁢                                                                          ⁢                  B                                                            2                ⁢                                  (                                      tp                    +                                                                  Line                        ⁢                                                                                                  ⁢                        B                                                                    Line                        ⁢                                                                                                  ⁢                        K                                                                              )                                                      -                                          1                                  Exp                  ⁢                                                                          ⁢                  K                                            ⁢                              exp                ⁡                                  (                                                            Exp                      ⁢                                                                                          ⁢                      K                      ×                      t                                        +                                          Exp                      ⁢                                                                                          ⁢                      B                                                        )                                                                                        (        5        )            
(e) the position P(X,Y)
                    {                                                            X                =                                                                            E                      1                                        +                                          E                      2                                                                                                  E                      1                                        +                                          E                      2                                        +                                          E                      3                                        +                                          E                      4                                                                                                                                              Y                =                                                                            E                      1                                        +                                          E                      3                                                                                                  E                      1                                        +                                          E                      2                                        +                                          E                      3                                        +                                          E                      4                                                                                                                              (        6        )            
where E1, E2, E3 and E4 are respectively energy values of the four angular signals forming the pulse, and
(f) the decay constant τ
                    τ        =                  -                      1                          Exp              ⁢                                                          ⁢              K                                                          (        7        )            